Why do Jem and Scout feel guilty when Mr. Avery tells them that children who disobey parents smoke cigarettes and make war on each other can cause a change in the seasons?

They felt guilty because Maycomb had not any recorded snowfall since 1885. THe little snow they dis get that year substantiated, according to Mr. Avery, that children behaving badly caused (among other bad things) bad weather. Being kids themselves, Scout and Jem give some credibility to Mr. Avery's claims.